This invention relates to negative electrodes for use in batteries (e.g., lithium electrodes for use in lithium-sulfur batteries). More particularly, this invention relates to methods of forming alkali metal electrodes having a thin barrier layer.
In theory, some alkali metal electrodes could provide very high energy density batteries. The low equivalent weight of lithium renders it particularly attractive as a battery electrode component. Lithium provides greater energy per volume than the traditional battery standards, nickel and cadmium. Unfortunately, almost no rechargeable lithium metal batteries have yet succeeded in the market place. Lithium metal battery technology has not approached its potential.
The failure of rechargeable lithium metal batteries is largely due to cell cycling problems. On repeated charge and discharge cycles, lithium xe2x80x9cdendritesxe2x80x9d gradually grow out from the lithium metal electrode, through the electrolyte, and ultimately contact the positive electrode. This causes an internal short circuit in the battery, rendering the battery unusable after a relatively few cycles. While cycling, lithium electrodes may also grow xe2x80x9cmossyxe2x80x9d deposits, which can dislodge from the negative electrode and thereby reduce the battery""s capacity.
To address lithium""s poor cycling behavior in liquid electrolyte systems, some researchers have proposed coating the electrolyte facing side of the lithium negative electrode with a xe2x80x9cbarrier layer.xe2x80x9d Such barrier layer must conduct lithium ions, but at the same time prevent contact between the lithium electrode surface and the bulk electrolyte. Many techniques for applying barrier layers have not succeeded.
Some contemplated lithium metal barrier layers are formed in situ by reaction between lithium metal and compounds in the cell""s electrolyte which contact the lithium. Most of these in situ films are grown by a controlled chemical reaction after the battery is assembled. Generally, such films have a porous morphology allowing some electrolyte to penetrate to the bare lithium metal surface. Thus, they fail to adequately protect the lithium electrode.
Various pre-formed lithium barrier layers have been contemplated. For example, U.S. Pat. No. 5,314,765 (issued to Bates on May 24, 1994) describes an ex situ technique for fabricating a lithium electrode containing a thin layer of sputtered lithium phosphorus oxynitride (xe2x80x9cLiPONxe2x80x9d) or related material. LiPON is a glassy single ion conductor (conducts lithium ion) which has been studied as a potential electrolyte for solid state lithium microbatteries that are fabricated on silicon and used to power integrated circuits (See U.S. Pat. Nos. 5,597,660, 5,567,210, 5,338,625, and 5,512,147, all issued to Bates et al.).
In both the in situ and ex situ techniques for fabricating a protected lithium electrode, one must start with a smooth clean source of lithium on which to deposit the barrier layer. Unfortunately, most commercially available lithium has a surface roughness that is on the same order as the thickness of the desired barrier layer. In other words, the lithium surface has bumps and crevices as large as or nearly as large as the thickness of the barrier layer. As a result, most contemplated deposition processes cannot form an adherent gap-free barrier layer on the lithium surface.
In addition, the high reactivity of lithium metal requires that lithium electrodes be fabricated in an environment free of oxygen, carbon dioxide, moisture, and nitrogen. These processing precautions add to the cost and difficulty in manufacturing suitable lithium metal electrodes.
For these reasons, lithium metal battery technology still lacks an effective mechanism for protecting lithium negative electrodes.
The present invention provides an improved method for forming active metal electrodes having barrier layers. Active metals include those metals that are highly reactive in ambient conditions and can benefit from a barrier layer when used as electrodes. The method involves fabricating a lithium electrode or other active metal electrode without depositing the barrier layer on a layer of metal. Rather the barrier layer is formed on a smooth substrate. A bonding or alloying layer is provided on top of the barrier layer, opposite the smooth substrate. Lithium or other active material is attached to the bonding layer to form the active metal electrode. A current collector may optionally be attached to the lithium or active metal during the process.
One aspect of the invention provides a method of fabricating an active metal electrode. The method may be characterized by the following sequence: (a) providing a barrier layer laminate and (b) bonding active metal to a barrier layer employed in the barrier layer laminate. The barrier layer laminate includes (i) a barrier layer disposed on a substrate and (ii) a bonding layer disposed on the barrier layer, the bonding layer being capable of forming a bond with the active metal. Preferably, the active metal is lithium or an alloy of lithium having a thickness of at least about 0.2 micrometers. In some cases, the lithium layer may be significantly thicker, on the order of millimeters. Layers of this thickness may be suitable for some primary cell electrodes.
In one embodiment, the substrate on which the barrier layer is disposed is a releasable web carrier including a layer of copper, tin, zinc, aluminum, iron, a polymeric material, or combination thereof. In a preferred embodiment, the substrate on which the barrier layer is disposed is an electrolyte such as a polymeric electrolyte. This approach has the advantage of producing a laminate that already contains both a negative electrode and the electrolyte. This product can be stored or handled and then bonded to a positive electrode to produce a laminated battery simply and efficiently. In a specific embodiment, the polymeric electrolyte is a polyalklyene oxide (such as a polyether), a polyimine, a polythioether, a polyphosphazene, a fluorinated polymer, or a polymer blend, polymer mixture, or copolymer thereof (e.g., a polyvinylidene-hexafluropropylene copolymer).
The barrier layer may be formed on the substrate by, for example, a physical deposition process or a chemical vapor deposition process. The resulting barrier layer should form a substantially impervious layer that is conductive to ions of the active metal. In one embodiment, the barrier layer is a glass layer that includes at least one of a lithium silicate, a lithium borate, a lithium aluminate, a lithium phosphate, a lithium phosphorus oxynitride, a lithium silicosulfide, a lithium borosulfide, a lithium aluminosulfide, and a lithium phosphosulfide. In an alternative embodiment, the barrier layer is made from an organic polymeric material such as a nitrogen or phosphorus containing polymer. In a specific embodiment, the barrier layer is a glass layer having a thickness of between about 50 angstroms and 5 micrometers, more preferably between about 500 angstroms and 2000 angstroms. Regardless of composition, the barrier layer preferably has an ionic conductivity of between about 10xe2x88x928 and about 10xe2x88x922 (ohm-cm)xe2x88x921.
In one embodiment, the bonding layer is not substantially reactive with moisture and air. For example, the bonding layer may be made from a metal such as aluminum, an aluminum alloy, silicon, zinc, manganese, and the like. It may also be made from an insertion compound such as carbon, spinel Mn2O4, NiO2, CoO2, VOx, pyrite (FeS2), MoO2, and the like. Still further, the bonding layer may be made from an organic material such as polyethylene oxide, polyethylene glycol, polyaniline, polyacetylene, polypyrrole, and the like (including crosslinked and copolymer forms). In a specific embodiment, the bonding layer is an aluminum or aluminum alloy layer having a thickness of at least about 100 angstroms.
In addition to the above-mentioned processing, the invention may optionally include attaching a current collector on the active metal to form a lithium laminate. This is then used in bonding the active metal to the bonding layer of the barrier layer laminate. In an alternative embodiment, bonding active metal to the barrier layer comprises bonding a free standing lithium layer to the barrier layer.
Regardless of whether the active metal is used alone or in conjunction with a current collector, bonding active metal to the barrier layer may involve pressing an active metal layer to the barrier layer laminate. This may be accomplished with a hot press, for example. In an alternative embodiment, bonding the active metal to the barrier layer involves evaporating or sputtering the active metal onto the barrier layer laminate.
Another aspect of the invention pertains to active metal electrodes formed by a method as outlined above. Batteries formed from such electrodes are also within the scope of this invention. One example of such batteries is a lithium-sulfur battery.
Yet another aspect of the invention provides a barrier layer laminate for use in fabricating an active metal electrode. The barrier layer laminate may be characterized by the following features: (i) a substantially smooth and flat substrate; (ii) a barrier layer disposed on the substrate, and (iii) a bonding layer disposed on the barrier layer. As explained above, the barrier layer should provide a substantially impervious layer which is conductive to ions of the active metal, and the bonding layer should be capable of forming a bond with the active metal. The properties and compositions of the components of layers comprising the laminate may be as described above.
These and other features of the invention will be further described and exemplified in the drawings and detailed description below.